[oe] need help: cdrkit & bitifields

[Frans Meulenbroeks]

2009/3/5 Koen Kooi <k.kooi at student.utwente.nl>:
> On 05-03-09 18:21, Frans Meulenbroeks wrote:
>
>> Ucbit is an unsigned char
>
> unsigned char makes me think of
> http://www.arm.linux.org.uk/docs/faqs/signedchar.php
>
> regards,
>
> Koen

Well it is explictly defined as unsigned char.
from utypes.h:

/*
 * The IBM AIX C-compiler seems to be the only compiler on the world
 * which does not allow to use unsigned char bit fields as a hint
 * for packed bit fields. Define a pesical type to avoid warnings.
 * The packed attribute is honored wit unsigned int in this case too.
 */
#if     defined(_AIX) && !defined(__GNUC__)

typedef unsigned int    Ucbit;

#else

typedef unsigned char   Ucbit;

#endif

The link you give is related to chars that are by default signed or unsigned

This one seems to be related to whether bitfields are ordered left to
right or right to left
(and if I recall correctly in the ansi C standard this is undefined
(but it has been a while since I checked)

Decided to do some testing. On x86: gcc 4.3.2:
frans at linux-suse:~/wt> cat tst.c
#include <stdio.h>

typedef struct {
    unsigned char bit0:1;
    unsigned char bits:6;
    unsigned char bit7:1;
} bitfields;

main(int argc, char **argv)
{
    char c = 0xf0;
    bitfields *bf = &c;

    printf("bit 0 = %d, bits = %x, bit7 = %x\n", bf->bit0, bf->bits, bf->bit7);
}
frans at linux-suse:~/wt> gcc  tst.c
tst.c: In function ‘main’:
tst.c:12: warning: initialization from incompatible pointer type
frans at linux-suse:~/wt> ./a.out
bit 0 = 0, bits = 38, bit7 = 1
frans at linux-suse:~/wt> gcc -O1 tst.c
tst.c: In function ‘main’:
tst.c:12: warning: initialization from incompatible pointer type
frans at linux-suse:~/wt> ./a.out
bit 0 = 0, bits = 38, bit7 = 1
frans at linux-suse:~/wt> gcc -O2 tst.c
tst.c: In function ‘main’:
tst.c:12: warning: initialization from incompatible pointer type
frans at linux-suse:~/wt> ./a.out
bit 0 = 0, bits = 1c, bit7 = 1
frans at linux-suse:~/wt> gcc -O3 tst.c
tst.c: In function ‘main’:
tst.c:12: warning: initialization from incompatible pointer type
frans at linux-suse:~/wt> ./a.out
bit 0 = 0, bits = 1c, bit7 = 1

Same test on beagle (gcc 4.3.1), -O0  -O1 -O2 -O3
root at beagleboard:~# ./tst0
bit 0 = 0, bits = 38, bit7 = 1
root at beagleboard:~# ./tst1
bit 0 = 0, bits = 38, bit7 = 1
root at beagleboard:~# ./tst2
bit 0 = 0, bits = 20, bit7 = 0
root at beagleboard:~# ./tst3
bit 0 = 0, bits = 20, bit7 = 0

Not sure what is happening here.
I tried to compile cdrkit with -O1 using
FULL_OPTIMIZATION = "-O1"
FULL_OPTIMIZATION_armv7a = "-O1"
BUILD_OPTIMIZATION = "${FULL_OPTIMIZATION}"
but that did not help either.

There is a compile time macro in gcc called BITS_BIG_ENDIAN but that
one is set to 0 for arm (same as for x86).

Puzzled....

Frans